​BREAST DENSITY (2)

BREAST DENSITY-UPPER

BREAST COMPOSITION

With regard to X-ray imaging, the breast can be divided into two basic tissues:

1. fat 

2. fibroglandular tissue  

The fibroglandular tissues have the same X-ray attenuation, but can be further subdivided, histologically, into 

a. the fibrous connective tissues that provide structure and support for the breast (Cooper’s ligaments)

b. the tubular branching milk ducts

c. the lobules that are the milk producing glands at the end of the ducts.  

There is always a layer of subcutaneous fat and then, essentially, individualized volumes of fat that are unique in distribution to each woman.  

The fibroglandular tissues are exactly that. The glandular tissues are the lobules at the ends of the milk ducts. The milk ducts going back from openings on the nipple, branch, to ultimately reach the terminal ducts that are capped off by the lobules. A “terminal duct” and its lobule are known as the “terminal duct lobular unit”(TDL). It is believed that most cancers originate in association with the TDL.

X-RAY ATTENUATION

Fat is “radiolucent”. X-rays pass through fat with little impediment. By convention, on mammography, fat is, essentially, black on the mammogram. All the other “fibroglandular” structures attenuate X-rays because they contain water and cast white or gray “shadows” on the mammogram. 

Since breast cancers are primarily cellular (water “density”) and rarely contain fat, their X-ray attenuation is similar to that produced by the fibroglandular tissues. This is a problem since a cancer that is surrounded by fibroglandular tissues on a mammogram, with no fat along its margin, will be, essentially, invisible on the mammogram unless it distorts the surrounding architecture, or produces calcium deposits that look suspicious on the mammogram.  

BREAST DENSITY AND DETECTING BREAST CANCER

Because fat in the breast (actually its major component in most women) is like “window glass” on a mammogram, breast cancers, which are almost completely cellular (usually with a minimum of fat, if any) are easily visible on X-rays when they are surrounded by fat. Even if they have a partial interface with fatty tissue, they are easily seen. When the breast tissues are heterogenous (fat and fibroglandular tissues mixed together) cancers are still fairly easily seen, but the risk is that they will be hidden in, or behind, these “islands” of fibroglandular breast tissue. When a breast has large volumes of dense breast tissue, the risk is that a cancer will be hidden. Digital Breast Tomosynthesis (DBT) greatly reduces the risk that a cancer will be hidden behind dense breast tissue, but if there is little if any fat abutting the cancer, it can even be hidden on DBT.

Some cancers in dense tissues are much more evident on Ultrasound which can differentiate cancers from the fibroglandular tissues in which they may develop. Magnetic Resonance Imaging (MRI), with intravenous contrast, is the best way to detect breast cancer regardless of tissue pattern.

BREAST “DENSITY”

In the 1970’s John Wolfe, a pioneer who helped develop Xeroradiography for mammography, divided the patterns of fat and fibroglandular (dense) tissues that he was seeing into 4 categories. His categories ranged from “all fat”, with little if any visible fibroglandular tissue, which he called “N1”, to his 4th pattern, “DY”, which he (incorrectly) termed “dysplastic” in which the breast contained major volumes of “extremely dense” fibroglandular tissues. This last category is a misnomer. There was no “dysplasia” involved. These prove to be nothing more than normal, water density tissues that are, predominantly, the fibrous supporting structures of Cooper’s ligaments intermixed with ducts and the glandular tissues. In some women these are extremely thick and produce “dense” breasts while in others they are fine to even nonexistent. In between was the “P1” pattern which was largely fat, but had scattered fibroglandular tissues. When the volume of fibroglandular tissues made the breast “heterogeneously” dense it was classified by Wolfe as a “P2” breast. These were his subjective evaluations–N1, P1, P2, DY.   

Wolfe was the first to suggest that the “density” of breast related to its risk of developing breast cancer ¹. In his analysis he (incorrectly) claimed that the DY pattern was almost 40 times as likely to develop breast cancer (see below).

BIRADS AND TISSUE PATTERNS

The American College of Radiology Breast Imaging Reporting and Data System has continued to recognize the various tissue patterns. To avoid confusion with Final Assessment categories, the most recent revision of BIRADS (ACR BI-RADS® Atlas 5th Edition) labels the four general breast tissue patterns as a, b, c, and d.

a. “The breast is almost entirely fat.”

b. “There are scattered areas of fibroglandular density

c. “The breasts are heterogeneously dense which may obscure small masses.”

d. “The breasts are extremely dense which lowers the sensitivity of mammography”

According to the BIRADS Atlas the percentages of women with each pattern are:

a. 10%

b. 40%

c. 40%

d. 10%

The most important problem revolving around tissue patterns is that cancers can be hidden on mammography in areas of dense tissues.

REFERENCES

1.Wolfe JN. Risk for breast cancer development determined by mammographic parenchymal pattern. Cancer. 1976 May;37(5):2486-92. doi: 10.1002/1097-0142(197605)37:5<2486::aid-cncr2820370542>3.0.co;2-8. PMID: 1260729.